1. Field of the Invention
The present invention relates generally to systems for detecting two different radiations, such as a beta particle and a gamma-ray or an alpha particle and an x-ray, and determining if they are coincident, that is, if they are detected within a suitably short time interval of one another. In one class of cases the two radiations are created by a single nuclear decay or similar event, so that this time interval is nanoseconds or less, meaning that “coincident” is essentially “simultaneous” from a practical point of view. In another class of events, the nuclear decay producing the first radiation also creates a nuclear excited state that decays with a characteristic half life τ, so that the time interval defining “coincident” is a few times τ.
More specifically, the invention relates to the application of “phoswich” detectors to making coincidence measurements either of the first class or of the second class in cases where τ is shorter than or comparable to the natural time constants of the detector system. “Phoswich” is a coined term in the art that is the concatenation of phosphor and sandwich. Phoswich is sometimes used as a noun and sometimes as a descriptor for such terms as “detector,” “detector assembly,” “scintillator,” “scintillator assembly,” or “transducer.” For simplicity, we will normally use phoswich as a noun.
The specific embodiments described relate to applying pulse shape recognition techniques to signals generated by a transducer assembly (or simply transducer) comprising a phoswich coupled to a photomultiplier tube (PMT) in order to determine whether, in a single detection event, the phoswich absorbed a first radiation, a second radiation, or both simultaneously. The techniques can also be applied when a photodiode replaces the PMT or when other detector systems entirely are employed. The application associated with the described specific embodiment, namely the detection of dilute radioactive Xenon (hereafter radio-Xenon) in atmospheric samples, is given particular attention only because this was the area in which the method was first developed.
The techniques that we have developed should therefore should not be construed as being limited to this specific application. Any detection system, for example, that produces output pulses whose time characteristics vary with type of detected event could be treated using the method.
2. A Synopsis of Current Prior Art
Coincident radiation detection is commonly used in a variety of nuclear and nuclear medicine measurement techniques. It is particularly powerful in the detection of rare events in the presence of significant background radiation when the desired event emits a pair of radiations. This is because, while random background events might mimic either member of the pair, the chance of randomly mimicking both scales as the product of the background rate's probability of mimicking either radiation times the length of the coincident window τC. Thus, ignoring detection efficiencies, if RR is the rare event rate that produces radiations 1 and 2, while RB1 and RB2 are the background rates at the two radiations, then the ratio ρ of RR to the “accidental” background rate RB12 from coincident background events is:
                    ρ        =                                            R              R                                      R                              B                ⁢                                                                  ⁢                12                                              =                                                    R                R                                                              R                                      B                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  R                                      B                    ⁢                                                                                  ⁢                    2                                                  ⁢                                  τ                  c                                                      .                                              (        1        )            
Thus, for example, both RB1 and RB2 can be 100 times as large as RR and ρ can still be 100 if τC is 1 microsecond, a fairly long coincidence inspection period.
In the past several years, scientists at Pacific Northwest National Laboratory (PNNL) have applied this technique to detecting radio-Xenon in atmospheric samples, an effort undertaken to develop instrumentation to support the International Nuclear Test Ban Treaty. Monitoring radio-Xenon in the atmosphere is one of several methods currently employed in the U.S. Atomic Energy Detection System program to detect nuclear weapons testing. Radio-Xenon is important in this context first because it is produced in significant amounts in a nuclear explosion; second, because, as a gas, it can escape from deep underground test sites; and third, because four radioactive isotopes are produced whose half life is sufficiently long that they can be detected far from the test site several days later. Even so, the amounts that would be present would be miniscule, even after prodigious concentration efforts. Estimates by PNNL scientists are that a statistically significant “signal” from a nuclear test might lie in the counts per minute to counts/hour range. At these levels background counting from nearby natural sources of radioactivity would completely overwhelm the radio-Xenon counts in the absence of the advantages conferred by coincident counting.
The current state of the art in radio-Xenon detection systems is an ARSA (Automated Radio-Xenon Sampler and Analyzer) system developed at Pacific Northwest National Laboratory [REEDER-1998, MCINTRYE-2001, REEDER-2004, RYNES-2004]. Because the radio-Xenon is greatly diluted by atmospheric mixing between the point of origin and the detection site, the system extracts all xenon from a large air volume and then measures its radioactivity in an extremely low background counter that is shown schematically in FIG. 1. The Xenon samples, typically only a few cc each, are placed in cylindrical cells 1 made of the fast plastic scintillator BC-404, each of which has a photomultiplier tube 2 (PMT) on either end. These cells are optically isolated 5 from and enclosed by pair of large NaI(T1) scintillator blocks 7 that are also optically isolated 5 from each other and each viewed by two large PMTs 8. The whole assembly is enclosed in a radiation shield (not shown) and housed in a Lead (Pb) cave to further reduce environmental background radiation.
The system's 12 PMT are all instrumented with preamplifiers, analog shaping amplifiers, multichannel analyzers, and time coincidence detection circuitry, all of which are standard commercial units known to those skilled in the art. When any PMT detects radiation, a test is made of the other PMTs to see if they also detect radiation. If a pair of small PMTs attached to a Xenon cell and a pair of large PMTs on the same NaI block all see light simultaneously, then the event is deemed valid and the electron energy is measured by summing the amplitudes of the pulses from the two small PMTs and the energy of the gamma-ray is measured by summing the amplitudes of the pulses from the two big PMTs.
The event is then added to a plot similar to the one shown in FIG. 2A [REEDER-2004], which displays the gamma-ray energy versus the electron or beta particle energy. Each horizontal bar represents a radio-Xenon decay mode that emits a gamma-ray of fixed energy (A=249.8 pkeV, B=81 keV, C=31 keV) in coincidence with a beta particle of fixed end point energy or a conversion electron of fixed energy. We will not go into the details here, but the different decay patterns are characteristic of different radio-Xenon isotopes,as described by by Reeder and McIntyre [REEDER-1998, MCINTRYE-2001]. As is well known in the Art, the narrower the gamma-ray lines are, the better the gamma-ray detector's energy resolution and the easier it is to detect a particular Xenon isotope against random background counts. FIG. 2B shows the gamma-ray spectra found by projecting the plot of FIG. 2A against the zero beta energy axis. All three lines resolve cleanly, with an achieved resolution of about 26% being reported at 81 keV [REEDER-2004].
While achieving high coincidence detection efficiency and acceptable energy resolutions for the gamma-ray lines, the current ARSA system has a number of drawbacks. In particular, while the ARSA system works well in a laboratory setting, it has not been easy to transfer its technology to an industrial manufacturer or particularly successful in field operation. Part of this lack of success stems from the complex electronics required to implement the required coincidence detections, part from the complex calibration procedures required to calibrate all 12 PMT gains, and part by the tendency of the PMT gains to drift with temperature and time. Because the ARSA system is intended for remote, unattended operation, a detector design that requires a regular, complex calibration is not acceptable.
Recognizing these issues, the PNNL scientists recently published a paper describing a new approach, indicated schematically in FIG. 3. [ELY-2003] In this detector system a cell of radio-Xenon 12 was presented to a phoswich comprising a 0.04″ thick CaF2(Eu) scintillator (940 ns decay constant) 13 coupled via a 0.25″ thick quartz optical window 15 to a 2″ by 2″ cylindrical NaI(T1) crystal (250 ns decay constant) 16. This phoswich assembly was then optically coupled to a single PMT 18. The figure does not show the required optical housing and radiation shielding that are well understood in the art. With these dimensions, the CaF2 scintillator stopped both conversion electrons and beta particles up to 900 KeV, while most x-rays and gamma-rays absorbed in the thicker NaI(T1) crystal. The PMT's anode output was connected to a charge integrating preamplifier 20 whose output is then fed into a fast digital signal processor 20 that captured and analyzed pulse waveforms from events in the detector. The signal processor measures the amplitude of the integrated preamplifier output pulses to determine the energy E of any detected event as is commonly done,but also took the unusual step of measuring the pulses' initial slopes S (or risetimes) as well.
FIG. 4 shows traces from two relatively energetic gamma-rays captured from the phoswich. The researchers proposed that radiation interactions in the phoswich could then be characterized according to their signal risetimes, with fast risetimes indicating interaction in NaI(T1) only, slow risetimes indicating interactions in CaF2 only, and intermediate risetimes indicating combination or coincident events. Therefore, for each event, they produce an (S, E) pair, which would then be plotted similarly to FIG. 2A. FIG. 5 shows a set of results from a radio-Xenon sample. The lower horizontal band of pulses at 700 ns risetimes corresponds to CaF2 only events from beta particle absorptions. The upper horizontal band at 1400 ns risetimes corresponds to NaI only events from gamma-ray absorptions. The “mixed” events corresponding to beta-gamma coincidences are the sloping bands that lie between the CaF2 only and NaI only bands.
Unfortunately, as FIG. 5 shows, the slope cannot be resolved accurately enough to distinguish the three different event types, particularly at low energies. Thus, while this method of pulse shape coincidence detection works well enough to distinguish the CaF2(Eu) only and NaI(T1) only events, the combination events—corresponding to the beta-gamma coincidences required for radio-Xenon monitoring, could only be poorly identified by this method and their energies could not be extracted with any accuracy. The authors therefore concluded that, even if an improved algorithm were developed, it would still be “challenging to separate the individual beta and gamma contributions of a single pulse with any precision” [ELY-2003]. Thus, while the approach of pulse shape coincidence detection showed potential for reducing the complexity of the original ARSA system, the PNNL researchers were disappointed to observer that its sensitivity and performance were insufficient to meet the requirements of the radio-Xenon monitoring application.
Therefore, for applications where one wished to sensitively detect two radiations in coincidence, such as the discussed beta-gamma coincidence, it would be beneficial to have a detector system that possesses the sensitivity of the ARSA system shown in FIG. 1 while also possessing the physical simplicity of the phoswich-based system shown in FIG. 3.